Any welding supply shop you might go to is going to have food-grade CO2. You can also easily get it from scientific lab suppliers or any number of other places,

Any welding supply shop you might go to is going to have food-grade CO2. You can also easily get it from scientific lab suppliers or any number of other places, and I've shopped at homebrew stores that have refill programs (you drop off your empty tank, and once a week the welding shop picks 'em up, refills 'em, and returns them to the store). Food-grade CO2 is dirt cheap and readily available.